The present invention relates generally to birefringence measurement apparatuses, and more particularly to a birefringence measurement apparatus that measures a birefringence of calcium fluoride (CaF2) to F2 laser, usable for an exposure apparatus that uses F2 laser.
A hyperfine pattern formation has increasingly been demanded with a recent progress of highly integrated semiconductor circuits. A demagnification projection exposure apparatus has frequently been used as a lithography apparatus to transfer a fine pattern. The higher integration requires increased resolution of a projection lens, which requires a shorter wavelength of exposure light and a larger numerical aperture of a projection lens.
The shortened wavelength of exposure light has advanced from a g-line (with a wavelength of 436 nm) to an ArF excimer laser (with 193 nm) through an i-line (with 365 nm) and a KrF excimer laser (with 248 nm), and use of an F2 excimer laser (with 157 nm) has been considered promising. A conventional optical element is applicable to an optical system for the wavelength range to the i-line, but conventional optical glass cannot be used for the wavelength range including the KrF and ArF excimer lasers and the F2 laser due to its law transmittance. Therefore, an optical system in an excimer laser exposure apparatus has commonly used an optical element made of quartz glass or calcium fluoride having larger transmittance to a shortened wavelength of light, and it has been considered that an F2 laser exposure apparatus necessarily uses an optical element made of calcium fluoride.
While each lens in a projection lens should be polished with ultimate surface precision, the lens when made of polycrystal causes the polishing speed to vary according to crystal orientations, and a difficulty in maintaining its surface precision. In addition, the polycrystal easily segregates impurities at a crystal interface, deteriorating uniformity of refractive index and emitting fluorescence responsive to a laser irradiation. For these reasons, a large aperture and highly homogeneous single crystal calcium fluoride have been demanded.
Calcium fluoride single crystal has been manufactured mainly by the crucible descent method or Bridgman method. This method fills highly purified materials of chemical compounds in a crucible, melts in a growth device, and gradually descends the crucible, thereby crystallizing the materials from the bottom of the crucible. The heat history in this growth process remains as a stress in calcium fluoride crystal. Calcium fluoride exhibits birefringence to the stress. The residual stress deteriorates the optical characteristics, and thus the heat process applies so as to remove the stress after the crystal growth. A birefringence measurement follows the heat process, and feeds the product to the next lens process step after confirming that the birefringence amount is less than the desired value.
The stress-dependent birefringence is a function of the stress and a piezo-optical coefficient. Since the piezo-optical coefficient is different according to wavelengths of light, the birefringence amount differs according to used wavelengths even under the same stress condition. Therefore, the birefringence amount of the calcium fluoride used for the F2 laser exposure apparatus should be measured with the F2 laser (with a wavelength of 157 nm).
However, the F2 laser is absorbed by oxygen and cannot transmit in the air, requiring a special environment without oxygen, and thus disadvantageously causing a large measurement apparatus, the increased cost, and the deteriorated operability.